model,py

class lalshah_items(models.Model):
    _name = 'lalshah_items.lalshah_items'
    _description = 'lalshah_items.lalshah_items'
    _rec_name = 'item_name'

    item_name = fields.Char(string="Item Name")
    item_description = fields.Text(string="Item Description")
    item_unit = fields.Char(string="Unit")
    item_hsn = fields.Integer(string="HSN")
    item_porate = fields.Float(string="PO Rate")
    item_gstrate = fields.Float(string="GST Rate")
    item_porate_inc_gstrate = fields.Float(string="PO Rate (Inc GST)")
    margin = fields.Float(string="Margin")
    ns_rate = fields.Float(string="NS Rate", compute='_compute_ns_rate', store=True)
    cs_rate = fields.Float(string="CS Rate")
    status = fields.Boolean(string="Status", required=True, default=True)
    updated_timestamp = fields.Datetime(string="Timestamp", default=lambda self: fields.datetime.now(), readonly=True)


    @api.depends('ns_rate')
    @api.onchange('item_porate', 'margin', 'ns_rate')
    def _compute_ns_rate(self):
        for record in self:
            record.ns_rate = float(math.ceil(float(record.ns_rate)*4)/4)